1. Field of the Invention
The present invention relates to a motor-driven mold clamping device for a motor-driven injection molding machine.
Motor-driven injection molding machines have gradually replaced hydraulic ones and increasingly been used in recent years. One reason lies in their simple configuration as compared with the hydraulic injection molding machines because of the non-necessity of a hydraulic pump, hydraulic tubes and valves. In addition, a servo motor used for the source of power allows the easier control of the motor-driven injection molding machines. The servo motors are used in most cases for an injection device and a mold clamping device.
2. Description of the Related Art
In the mold clamping device, a toggle system is often used. The toggle system uses a toggle mechanism to double the force generated by the servo motor which is then transmitted to a mold by means of a toggle link. The mold clamping devices of the type described are undergoing changes and refinements. An example of an improved mold clamping device is disclosed in Japanese Patent Publication No. 3-56654 (56654/1991). The disclosed mold clamping device is applied to an injection molding machine employing the toggle mechanism driven by the servo motor to move a movable platen for the mold clamping. The toggle mechanism has a crosshead which is associated with the movable platen. The mold clamping device comprises a crosshead position detector for detecting a position of the crosshead in the toggle mechanism. More specifically, the crosshead has a stroke for a single cycle of the mold clamping operation. The stroke is divided into a plurality of regions. The mold clamping device also comprises a region determination unit for determining the region in which the crosshead is located upon the reception of an output from the crosshead position detector. Furthermore, the mold clamping device comprises a movable platen position calculating unit for use in approximating, as a linear equation, a correlation between the crosshead positions (regions in which. the crosshead is located) and the positions of the movable platen to calculate the xe2x80x9ccurrentxe2x80x9d position of the movable platen by using the linear equation from the outputs of the region determination unit and the crosshead. position detector. A display unit displays the position of the movable platen calculated by the movable platen position calculating unit. The above-mentioned configuration provides the position of the movable platen calculated by the approximation with the linear equation discussed above.
However, the correlation between the positions of the crosshead and the movable platen is non-linear with a curved profile rather than linear. In this case, if a curve is divided into a plurality of regions, the linear approximation of the plurality of regions originated from the curve results in an error. Thus, the above-mentioned technique has an inherent problem of a difference between the calculated position and the position where the platen should be located. This means that the above-mentioned mold clamping device has some limitations in control accuracy for mold opening and closing positions.
Therefore, an object of the present invention is to improve the accuracy of controlling mold opening and closing positions.
The present invention is applied to a motor-driven mold clamping device comprising a servo motor, a movable platen, and a toggle mechanism operated by the servo motor to advance and retract the movable platen for clamping a mold.
According to an aspect of the present invention, the motor-driven mold clamping device comprises a setting unit for use in setting a position of the movable platen, a position detector for detecting a position of a crosshead in the toggle mechanism, and a control unit. The control unit approximates a correlation between the positions of the crosshead and the movable platen by using at least one function of a quadratic or higher (e.g., cubic, quartic, etc.) order. The control unit calculates the position of the crosshead by means of the approximated function with the position of the movable platen set by using the setting unit. The control unit then controls the position of the crosshead for the control of the opening and closing of the mold, the control being made in accordance with the detected position supplied from the position detector and the calculated crosshead position.